The present proposal assesses the stress and strain of a warm thermal environment in terms of the various physical and physiological factors involved. The entire physics of heat transfer between the skin surface and the environment can be characterized by a single factor Psi, defined as the ratio of the effective sensible heat transfer coefficient (for radiation, convection and conductance) to the insensible heat transfer coefficient (by evaporation). Psi is primarily a function of clothing insulation worn, air movement and barometric pressure. The ratio of Psi/W, where w is the fraction of the total body skin surface wet with sweat, serves as a biophysical index of the accompanying regulatory strain necessary for thermal equilibrium at any given temperature and ambient vapor pressure. On a psychrometric chart (vapor pressure on ordinate; temperature on abscissa) Psi/w is the negative slop of a locus of environmental conditions for which indicators of physiological strain, such as averaged skin and internal body temperature and heart rate, are postulated as equivalent. The temperature at the intersection of this locus with the 50% rh curve serves as a single Effective Temperature (ET*) index of the total environment. The proposed research seeks to compare and make an adequate assessment of various empirical heat stress indices and ET to such physiological responses as cardiac output, blood pressure, heart rate and sweating rate. Heat-acclimated or exercise trained males and females between the ages of 18 to 45 years clothed and unclothed will be studied during exposure to high operative temperature, humidity and during steady and intermittent positive and negative work. Additionally, we shall quantify to what extent local area cooling systems, using evaporatively cooled air, increase heat tolerance limits based on empirical indices.